1. Field of the Invention
The present invention relates to a solid image pick-up device to be used as an input unit for a camera-integrated VTR (Video Tape Recorder), an electronic still camera or the like, and relates more particularly to a solid image pick-up device having an unnecessary charge discharging section formed adjacent to a horizontal charge transfer section.
2. Description of the Related Art
Recently, there has been progressed a diversification of a solid image pick-up device to be used as an input unit for a camera-integrated VTR. Further, a solid image pick-up device has come to be used as an input unit for an electronic still camera for producing a hard copy or an electronic still camera for producing a picture for observation on the screen, by converting optical information into an electric signal and storing this electric signal in a recording medium, in stead of exposing a film.
Such a solid image pick-up device has a photoelectric conversion section for converting an optical signal into an electric signal and a charge transfer section for transferring a signal charge stored in the photoelectric conversion section into a vertical direction and a horizontal direction. However, in addition to a signal charge which is essential as a video signal, there also exist unnecessary signal charges such as a charge photoelectrically converted during an unnecessary period and a charge due to a current generated from a silicon oxide film interface.
When a solid image pick-up device is used as an input unit for a camera-integrated VTR, unnecessary signal charges are settled at an insignificant level after images of a few screens are displayed, causing no large problem. However, when the solid image pick-up device is used as an input unit for an electronic still camera, unnecessary signal charges are superimposed on a signal charge as a primary video signal, thus causing a deterioration in the picture quality.
Further, in the case of taking time for removing an unnecessary signal charge, a time delay is generated until a shutter operation is actually performed after a shutter button is pressed to transmit a signal to take a shuttering action. As a result, there is a problem that a shutter chance may be lost.
Accordingly, unlike the case of using a solid image pick-up device as an input unit for a camera-integrated VTR, the solid image pick-up device when used as an input unit for an electronic still camera needs to remove instantaneously all the unnecessary signal charges existing in the photoelectric conversion section and the vertical and horizontal charge transfer sections at the same time when the shutter button is pressed to transmit a shuttering signal.
As a method for removing unnecessary charges, there has been described a method for removing unnecessary charges existing in the photoelectric conversion section in, for example, xe2x80x9cVertical Overflow Structure CCD Image Sensorxe2x80x9d, by Ishihara et al., a publication of the Institute of Television Engineers of Japan, Vol. 37, No. 10 (1983), pp. 782-787. This is a method for forming a low density Pxe2x88x92-type semiconductor area beneath an N-type semiconductor area for structuring a photoelectric conversion section and applying an inverse bias voltage to an N-type semiconductor substrate to deplete the Pxe2x88x92-type semiconductor area. By this method, a vertical overflow drain structure is formed for restricting a blooming by moving a surplus charge to the N-type semiconductor substrate and removing all the signal charge to the N-type semiconductor substrate, thereby removing the unnecessary charges existing in the photoelectric conversion section.
Further, since a high-speed operation is possible for a horizontal charge transfer section, as a method for removing unnecessary charges existing in the horizontal charge transfer section, there is employed a method for moving the unnecessary charges to a reset drain provided at the end of the horizontal charge transfer section by a normal operation.
On the other hand, as a method for removing unnecessary charges existing in a vertical charge transfer section, there is a method for transferring the unnecessary charges of the vertical charge transfer section to an unnecessary charge discharging section in a forward direction.
However, according the conventional solid image pick-up device, it is not possible to remove efficiently the unnecessary charges existing in the vertical charge transfer section.
It is an object of the present invention to provide a solid image pick-up device having an unnecessary charge discharging section which can remove efficiently unnecessary charges existing in a vertical charge transfer section.
A solid image pick-up device having an unnecessary charge discharging section according to the present invention comprises a semiconductor substrate of first conductive type. A semiconductor well layer of second conductive type is formed at the surface of the semiconductor substrate. First device isolation areas of second conductive type are formed at the surface of the semiconductor well layer. A plurality of photoelectric conversion sections are formed at the first device isolation areas. The photoelectric conversion sections are converting optical information into electric signals. There are formed a vertical charge transfer section for transferring charges in a vertical direction out of the electric signals converted by the photoelectric conversion sections and a horizontal charge transfer section for transferring charges in a horizontal direction out of the electric signals converted by the photoelectric conversion sections. A second device isolation area of second conductive type is formed in contact with the first device isolation areas. The second device isolation area has an impurity density higher than that of the semiconductor well layer. An output circuit section is formed at the second device isolation area. The output circuit section outputs charges transferred by the vertical charge transfer section and the horizontal charge transfer section. An unnecessary charge discharging section is formed at a side of the horizontal charge transfer section opposite to the vertical charge transfer section. The unnecessary charge discharging section discharges unnecessary charges of the vertical charge transfer section and the horizontal charge transfer section. A third device isolation area of second conductive type is formed in contact with the second device isolation area between the horizontal charge transfer section and the unnecessary charge discharging section. The third device isolation area has an impurity density higher than that of the semiconductor well layer. A potential barrier section determines electric potential between the horizontal charge transfer section and the unnecessary charge discharging section.
This solid image pick-up device may have an insulation film formed on the potential barrier section and the unnecessary charge discharging section, and a horizontal charge transfer electrode formed on the insulation film.
A semiconductor area of first conductive type structuring a buried channel of the horizontal charge transfer section may be formed at the surface of the semiconductor substrate in the area where the unnecessary charge discharging section is formed. Further, the impurity density of the first device isolation areas may be equal to that of the third device isolation area.
It is preferable that a potential difference between the voltage applied to the unnecessary charge discharging section and the electric potential of the potential barrier section is 0.5 V or above. However, this potential difference does not need to be 0.5 V or above.
Further, the third device isolation area may have a projection extending to the second device isolation area. The projection may be in contact with the second device isolation area. Further, the second device isolation area may have a projection extending to the third device isolation area. The projection may be in contact with the third device isolation area.
A plurality of the third device isolation areas may be separated in a direction to which the horizontal charge transfer section extends. The electric potential of the potential barrier section may be determined by narrow channel effect of said third device isolation area. A reference voltage may be supplied to the second device isolation area.
According to the present invention, as the third device isolation area for determining the electric potential of the potential barrier section and the second device isolation area in which the output circuit section is formed are connected together. The second device isolation area and the third device isolation area have impurity density higher than that of the semiconductor well layer. Accordingly, it is possible to supply an electric potential directly to the third device isolation areas through the low-resistance second device isolation area when a voltage is applied to the electrode of the horizontal charge transfer section. Therefore, a solid image pick-up device which can remove efficiently unnecessary charges existing in a vertical charge transfer section is provided. Also, it is possible to restrict the electric potential of the third device isolation area for determining the electric potential of the potential barrier section from being modulated, thus stabilizing the electric potential. As a result, according to the present invention, it is possible to prevent a reduction of the charge transfer capacity of the horizontal charge transfer section determined by the difference between the electric potential of a charge storage area of the horizontal charge transfer section and the electric potential of the potential barrier section.